Acoustic device



April 23, 1957 F. B. DANIELS 2,789,651

ACOUSTIC DEVICE Original Filed Sept. 5. 1950 INVENTOR, FRED 8. DA N/ELS.

F/G. 5 BY f f A T TORNE Y.

Unit d St te P t a represented by the Secretary of 183,221, now Patent No. 2,739,659, dated March 27,

1956. Divided and Serial N0." 539,703

2 Claims. 01. 181-55 (Granted under 'Iitle 35, U, S; Code-(19 52), sec. 266) this application October 10, 1955,

The invention described herein may be manufactured nd Z'used; by; or for the Governmentv for governmental purposes without the payment of any royalty thereon.'

This invention .relatesto. an. electro -acoustic'al apparatu'siand mcreparticularly to a' device for translating acoustical waves into..electrical. variations, and includes an acoustical transmission line of novel design. This invention relates particularly to a highly directional device that is relatively insensitive to interference from wind and other air turbulences.

There are several types of acoustical impedance elements for use in conjunction with microphones, those shown in Patent No. 2,228,886 to H. F. Olson being representative of the existing types that most nearly resemble this invention and which generally consist of a plurality of input tubes of varying length coupled to a common electro-acoustical transducer and terminating in some sort of acoustical impedance.

It is an object of this invention to provide an acoustical coupling device for an electro-acoustical transducer.

It is a further object of this invention to provide an acoustical coupling device that is highly directional.

It is a further object of this invention to provide an acoustical coupling device that does not require an additional terminating acoustical impedance, for use with an electro-acoustical transducer.

It is a further object of this invention to provide an acoustical coupling device that is relatively compact and easily constructed.

This device is particularly valuable in sound ranging systems where the eddy characteristics of the wind at the electro-acoustical transducer closely resemble the muzzle wave that it is desired to detect.

The invention and the foregoing as well as other features thereof will be understood more clearly from the following detailed description and with reference to the accompanying drawings in which:

Figs. 1 to 4 show sectional views through the axis of various species of this invention, and

Fig. 5 shows a projection of one exterior surface of another specie.

Referring now to the drawings, wherein similar refer- Red Bank, N. J., .assign or to the United 1 2,789,651 Paten ed Apr. 2,3. 19,51.

scribedimpedance, a terminating impedance such as a.

' damped pipe, would be necessary, and this would add considerably "to the length, weight and cost of manufacture of the device. Besides, the commonly used damp ing materials that such a damped pipe-.\vould,require-- I might absorb'moisture, become displaced, or otherwise:

be rendered ineffectual. A preferred form of this invention is seen in Fig. 2

where the pipes 1316 of Fig. 1 are replaced by a single pipe 110 of successively increasing sections 113, 114,

115 and 116. Plugs 117-120 are mounted near the.

junctions between the sections, and the holes 121124' in these plugs are, again, of suitable dimensions to match each section 'of the pipe individually and as a whole. This impedance matching is achieved by making where R is theacoustical resistance of the plug, :1 is the density of'air, c is the velocityof sound in air, Snflthe cross-sectional area of that section of pipe to theright of the junction and 'Sn'is the cross-sec'tionabarea of that ence numbers designate corresponding parts throughout,

Fig. 1 shows an acoustical coupling device 16 coupled to an electro-acoustical transducer 11 in a chamber 12. The acoustical coupling device 10 which is particularly designed to have directional and other desirable features, here consists of a series of tubular pipes 13, 14, 15 and 16, one end of each connecting to the chamber 12 and the other end of each terminating in plugs 17, 18, 19 and 20. These plugs have vents 21, 22, 23 and 24 of the size and shape necessary to match the characteristic impedance of their respective pipes. Since each pipe is, then, terminated by its characteristic impedance at the input end, no additional terminating impedance beyond the chamber 12 is required. Without these plugs of pre section of pipe to the left of the junction all quantities being expressed in c. g. s. units. In other words, as a result of satisfying this condition, a wave traveling from right to left in the tube would experience no reflection at the junctions. The pipe couples to chamber 112 that contains an electro-mechanical device 111, such as a con ventional pressure microphone. If physical dimensions permit, the device 111 may be mounted directly in the end of the pipe and the chamber 112 may be replaced by a flat cover.

This device is particularly suited to the pressure type microphone, which is cheaper, more rugged, and more suitable for low frequencies than the velocity type micro phone.

In operation, the sound waves entering the openings 121-124 are reinforced in the direction of the axis of the pipe, since, for example, a sound pulse traveling through the air along the outside of the pipe will enter through each successive opening 121-124 exactly in phase with the pulse traveling through the pipe, while the sound waves approaching from other angles will be more or less neutralized by the phase difference between a pulse traveling through the inside of the pipe and the successive pulses from the openings produced by the same wave on the outside of the pipe, thus making the device directional.

The apparatus of Fig. 2 can be easily and economically constructed since it can be made of ordinary water pipe of successively increasing sizes. This device can be made of considerable size. For example, one device was made 1980 feet long with 99 openings. Also, it may be paralleled with similar devices.

The wind disturbances in the form of eddy currents and other localized pressure variations are minimized to a large degree by this device since the wind interference is local and comparatively static while a sound wave exists only in motion. The effect of the wind is local and random while a sound wave arrives at the various openings with a definite phase relationship. A local pressure variation eifecting only a few openings would have a cumulative effect negligible compared to that of a sound wave coming from the right direction and acting on all of the openings. This noise reducing effect is very valuable in sound ranging where the effect of a local pressure change due to wind currents on a microphone closely resembles the pressure change due to a muzzle wave.

Another variation of this device, shown in Fig. 4 would have holes 321-324 drilled in the pipes 313-316 and the pipe lengths or hole sizes and numbers chosen to meet the basic requirement of this invention, that the openings and pipes form a balanced acoustical unit not requiring additional termination.

Another species, of this invention is seen in Fig. 3 where the coupledv sections of pipe of increasing diameter are replaced by a parabolic horn 210 of equivalent length and volume. This would be easily accomplished for relatively. short structures. In this species, the plugs 217'220 and end openings 212224 could be replaced by drilled holes of similar impedance as in Fig. 4, or a continuous slot 425, shown in Fig. 5, whose impedance per unit length balances the volumetric change so that the pipe 410 can be coupled to the electro-acoustical member without additional acoustical termination. In this case the reciprocal of the acoustical resistance of the slot per unit length must be equal to the change per unit length of the reciprocal of the characteristic impedance of the horn.

A portion of' the subject matter disclosed herein is claimed in the copending application of Fred B. Daniels, Serial No. 183,221, filed September 5, 1950, for Acoustic Dev-ice, now Patent No. 2,739,659, issued March 27, 1956, of which present application is a division.

What is claimed is:

l. A microphone comprising a container having a 25 paraboloidal surface, an elongated opening in said surface extending in the direction of the axis of said paraboloidal surface, the reciprocal of the acoustical resistance of the opening per unit length being equal to the change per unit length in thereciprocal of the characteristic impedance of said container.

2. An acoustical devicecomprising a tapered tubular pipe, the larger end of said pipe being closed acoustically, a continous slot extending, from the larger end of said tapered tubular pipe to the smaller end, said opening forming acoustical resistance, the reciprocal of the acoustical resistance of the slot per unit length of pipe being equal to the change per unit length in the reciprocal of the characteristic impedance of 'said pipe, and a transducer element mounted in the larger end of said tubular pipe.

References Cited in the file of this patent UNITED STATES. PATENTS 

